Heretofore, there is known a dynamic damper mounted on a rotational shaft such as a drive shaft or a propeller shaft of an automobile, for damping hazardous vibrations which should not be caused, such as flexural vibrations, torsional vibrations, etc. that are developed due to an unbalanced rotational behavior caused when the rotational shaft rotates.
The dynamic damper has a function to absorb the vibrational energy of the rotational shaft by converting the vibrational energy into vibrational energy of the dynamic damper by way of resonance, with the natural frequency of the dynamic damper being equal to the dominant frequency of excited hazardous vibrations of the rotational shaft.
One dynamic damper of the above type, which is disclosed in Patent Document 1, for example, comprises a boss having a center hole with a rotational shaft such as a drive shaft press-fitted therein, a ring-shaped weight (a mass portion) disposed concentrically with the boss, an elastic connector for connecting the boss and the weight radially, and a fixing band tightened on the outer circumferential surface of the boss to fix the boss to the rotational shaft.
Further, a dynamic damper disclosed in Patent Document 2 comprises a boss having locking grooves at both axial ends, a ring-shaped mass portion disposed around the boss, and fixing bands wound on the locking grooves for fixing the boss.
However, the dynamic dampers disclosed in Patent Documents 1 and 2 are disadvantageous in that water may penetrate into a clearance between the rotational shaft and the inner wall surface of the boss in a portion where the fixing band is not tightened on the boss.
The dynamic damper mounted on the rotational shaft such as a drive shaft or propeller shaft is generally used in a harsh air environment near the surface of the ground. When the dynamic damper is used for a long period under such an environment, a material such as rubber fatigues, and its tightening force on the rotational shaft is lowered. As a result, water can penetrate into the clearance between the rotational shaft and the inner wall of the boss in the portion where the fixing band is not tightened on the boss.
The above problem can be solved by reducing the inner diameter of the boss to improve the contact between the inner wall surface of the boss and the outer circumferential surface of the rotational shaft. However, in this case, an increased force is required to press-fit the rotational shaft into the boss against the inner wall surface thereof when the dynamic damper is mounted on the rotational shaft. Thus, the working efficiency is reduced, and an additional press-fit apparatus or the like is required other than existing facilities, resulting in higher production costs.
Further, when the increased force is applied to press-fit the rotational shaft into the boss, against the inner wall surface thereof the inner wall of the boss is excessively stressed and deformed. Particularly, in the boss, the portion to be tightened by the fixing band is thinner than the other portions, and thereby is deformed and extended in the direction of pressing the rotational shaft.
Patent Document 1: Japanese Laid-Open Utility Model Publication No. 59-003041
Patent Document 2: Japanese Laid-Open Patent Publication No. 02-154827